Many cable networks are built in a cascaded (tree and branch) structure. This means that several amplifiers and taps are all placed in series with branches tapping off some of the signal and feeding again a cascade of amplifiers and taps. These branches can be tapped off to feed another cascade of taps. Since taps are placed in series and therefore have different input signal levels caused by attenuation in the coaxial cable and in the taps itself, different models of taps with different tap loss values are used. Usually, the first taps have a high input signal level and therefore need to have a high attenuation from input to tap output port, known as tap loss, and automatically a low insertion loss from the input to the output. When migrating down the line, the tap loss needs to be lower as there is less energy because of loss in the former taps and in the coaxial cable and the insertion loss becomes automatically higher as more energy is tapped off from the line.
These taps are known in the industry as “outdoor taps” as such a network is typically not mounted in cabinets but on overhead strands or poles or on the walls of houses.
In such a network, a small deviation from the ideal frequency response from in to out (so called ripple) in the outdoor taps is amplified by the total number of outdoor taps placed in series. This means that, for example, a small and seemingly unimportant ripple in the frequency response of 0.2 dB in a single outdoor tap is multiplied to a more significant 2 dB when 10 outdoor taps are cascaded. Since the outdoor taps usually have a more or less equal frequency response, this is a real problem.
The same is true for the insertion loss from in to out. If the insertion loss can be lowered with only as little as 0.1 dB this means that at the end of the coaxial line the level of insertion loss will be 0.1 dB×the number of cascaded outdoor taps higher. This is of great importance as many networks need to be stretched to higher frequencies to transport more and more data and programs. Higher frequencies mean significantly more loss in the coaxial cables and thus lower levels at the end of the line. Re-spacing or adding of amplifiers placed in the cascaded network is usually not possible or only at very high costs. A lower loss in the outdoor taps therefore is a real advantage.